The present invention relates to an elastic epoxy resin hardener system. Epoxy resins, in particular those which are prepared from bisphenol A and epichlorohydrin, are known raw materials for the preparation of high-quality casting resins, coating compositions and adhesives. These epoxy resins may be cured by means of polyamines and possess, besides good chemical and solvent resistance, good adhesion to many substrates. The usability of these resin/hardener systems is often limited by insufficient elasticity or flexibility in the crosslinked state. In particular for applications in which temperature-change stresses have to be taken up by a high elasticity of the coating materials, the elasticity of the unmodified standard epoxy resin systems is insufficient. In the adhesives sector, there is a need for epoxy resin systems which are still sufficiently elastic at low temperatures, i.e. below 0.degree. C. In the automobile industry, use is made of epoxy resin adhesives which are only slightly flexible in the cured state. Although the adhesive bonds obtained with these resins have a high tensile shear strength, they easily separate by peeling under a lateral force. Adhesives which allow high tensile shear strengths to be achieved frequently give only a low angle peeling strength.
In adhesive bonding of metal sheets in the automobile industry, a procedure frequently used during basic body construction is first to apply the adhesive warm to oiled bright metal sheets, but not to cure it at this stage. On cooling, the adhesive solidifies. In the manufacturing process the prefabricated parts or the basic vehicle bodies are treated to remove the rolling grease in washing, phosphating and cataphoretic immersion-finish baths prior to curing in an oven. There is, therefore, a need for adhesives which have sufficient wash-out resistance even prior to curing. In particular, the cured adhesive system has to sufficiently protect the bright metal sheet against corrosion under the point of adhesion.
In principle, the elasticity of epoxy resin systems can be increased externally by addition of plasticizer or internally by reducing the crosslinking density. External elasticizing agents are not reactive and are not incorporated into the thermoset network. This type of modification is limited to specific application areas, since it has a series of disadvantages. These additives lead to considerable disruption of the thermoset structure, are limited in their plasticizing effect at low temperatures, tend to sweat out on thermal stressing and aging and result in cured systems that are brittle. To internally increase the elasticity, additions are made of compounds which react with the epoxy resins or hardeners and are incorporated in the crosslinking. In detail, the elasticizing action is achieved by incorporation of long-chain aliphatic or strongly branched additives into the resin or hardener component.
Vazirani (Adhesives Age, Oct. 1980, pp. 31-35) describes flexible single-component and two-component epoxy resin systems based on polyoxypropylenediamines and polyoxypropylenetriamines. The curing of the single-component system is carried out using dicyandiamide.
EP 0 354 498 (DE 3 827 626) discloses a reactive melt adhesive which contains a resin component, at least one thermally activatable latent hardener for the resin component and, if desired, accelerators, fillers, thixotropes and further customary additives. The resin component is obtained by reaction of
(a) an epoxy resin which is solid at room temperature and PA1 (b) an epoxy resin which is liquid at room temperature together with PA1 (c) a linear polyoxypropylene having amino end groups. PA1 (A) a compound that contains at least two 1,2-epoxide groups and that is a reaction product of: PA1 (B) a hardener.
The resins which are solid at room temperature are ones which for further processing have to be heated to above 50.degree. C. so as to lower the viscosity sufficiently for incorporation of further constituents of the melt adhesive to be made possible. In the reaction of the epoxy resins with the linear polyoxypropylene having amino end groups, a large excess of epoxide groups, based on the amino groups, is required so that the amino groups are completely reacted. A 5-fold to 10-fold excess is typical.
WO 93/00381 describes a further development of EP 0 354 498 that has improved low-temperature properties. The amino component comprises linear amino-terminated polyethylene glycols or linear and/or trifunctional amino-terminated polypropylene glycols.
EP 0 109 174 describes an epoxy resin composition that comprises (A) a polyepoxide and (B) a hardener, wherein the polyepoxide has been reacted with from 50 to 70% by weight of a polyoxyalkylenemonoamine that has a molecular weight of from 900 to 2000 g/mol. The resin-hardener mixture described in this document can be used as a flexible adhesive in the form of a single-component or two-component system. It has a low viscosity and can therefore be used without addition of solvent. It is disclosed that only one series of adducts, namely those prepared from .RTM.Jeffamine M-1000 (Texaco), shows a uniformly low viscosity independent of the amine content. In addition, it is pointed out that compositions containing less than 50% of polyoxyalkylenemonoamine have a relatively low flexibility at a high viscosity and compositions containing more than 70% of polyoxyalkylenemonoamine have relatively low adhesive strength and decreasing viscosity.
U.S. Pat. No. 4,423,170 describes water-dilutable epoxy resin compositions which comprise diepoxides that are obtained by partial reaction of diepoxides with polyoxyalkylenamines that have a molecular weight of from 900 to 2500 g/mol, and a latent hardener in aqueous medium. According to this document, epoxy resins are brought into a water-compatible form via incorporation of polyoxyalkylenepolyamines and are used in combination with latent hardeners as a single-component adhesive system. The polyoxyalkylenepolyamines are, in particular, polyoxyalkylenediamines and polyoxyalkylenemonoamines. The patent indicates that the polyoxyalkyleneamines have to be predominantly polyoxyethyleneamines to achieve sufficient solubility of the epoxy resin system in water (column 3, lines 23 to 26). Polyoxypropyleneamines do not achieve the objective of the patent (Example V), viz. the provision of water-dilutable systems.
The polyoxyalkylenemonoamines that have advantageous properties are described in this document as having the formula (column 2, lines 60 to 67) EQU CH.sub.3 -(O-CH.sub.2 -CH.sub.2).sub.r -[O-CH.sub.2 -CH(CH.sub.3)].sub.S -NH.sub.2,
where r is from 16 to 25 and s is from 1 to 6.